1. Field of the Invention
The present invention relates to a modified polyimide and a curable resin composition. More specifically, the present invention relates to a curable resin composition for the production of a more colorless transparent polyimide film.
2. Description of the Related Art
Organic films have the advantages of higher bendability, less brittleness, and lighter weight than glass. With the recent trend towards flexible displays, substrates for flat panel displays have been replaced with organic films.
Transparent polymer plastics are advantageous in terms of ease of processing, mass productivity, and price. Due to these advantages, transparent polymer plastics are widely used at present as materials for cover windows and touch panels for the protection of windows of flat panel display devices.
Like films for displays, films for touch panel substrates are also optical components for better visibility of LCD and OLED screens and are thus required to have low haze and high transmittance. Further, films for touch panel substrates should be highly durable under various environmental conditions as well as against fingernails due to the frequent finger touch. For high durability, transparent polymer plastics need surface treatment with high-hardness hard coating. High surface hardness, impact resistance, and flex resistance are requirements for the manufacture of high hardness products. PET or cyclo-olefin polymer (COP) films are currently in use for touch panel substrates. However, high retardation of PET causes diffuse reflection of incident light, making screen images invisible. High deposition temperature is required to lower the resistance of ITO in touch panels. However, the deposition temperatures of some COP substrate films are difficult to increase due to their poor heat resistance. That is, there are no materials that can simultaneously meet three requirements: excellent heat resistance, optical properties, and mechanical properties for high processability.
In recent years, polymethyl methacrylate (PMMA) resin sheets have been used for outer windows of portable display devices in place of transparent glass substrates. Due to their poor impact resistance, however, PMMA resin sheets tend to be brittle even when small external impacts are applied thereto. Some transparent sheets for outer windows of potable display devices are produced by coextrusion of a PMMA resin with a polycarbonate (PC) resin. The polycarbonate (PC) resin imparts impact resistance to the transparent sheets. However, the outer windows are pushed back when pushed down with a finger because of relatively low flexural modulus of the sheets.
On the other hand, cyclo-olefin polymer (COP) films exhibit excellent characteristics in terms of transparency, hygroscopicity, etc., but they have relatively low flexibility, heat resistance, and surface hardness. Particularly, relatively low surface hardness of cyclo-olefin polymer (COP) films leads to low scratch resistance. That is, cyclo-olefin polymer (COP) films are not sufficiently protected from scratches.
In attempts to solve such problems, many methods have been developed for producing flexible films in which transparent resin layers with good heat resistance and high strength are laminated. However, the lamination of multiple resin layers is inefficient in terms of processing and problems may arise from poor adhesion between the resin layers.
Some processes, such as evaporation deposition and sputtering, are essential for the production of transparent electrodes. Since such processes are carried out at high temperatures of at least 200° C., heat resistance of flexible substrates is considered an important factor in the production of transparent electrodes. Fillers may be added to maintain the basic mechanical properties (e.g., heat resistance) of flexible substrates. In this case, however, voids may be formed on the surface of substrates, resulting in poor hygroscopicity. Particularly, high dielectric constant fillers are used to improve the heat resistance of substrates but may deteriorate the adhesion of substrates to copper foils. When polymeric materials containing fillers are used for substrates, the substrate materials lose their flexibility, tending to be brittle. A method for improving the strength, heat resistance, and adhesiveness of a substrate is known in which a glass fiber is used to make a polymeric material into a prepreg. However, this method is difficult to use in applications where high dielectric constant is needed because the polymeric material loses its dielectric constant.
Hard coating agents are coated on substrates, dried, and UV cured to form hard coatings. Such hard coating agents include a polyfunctional acrylic oligomer, a polyfunctional acrylic monomer, a photoinitiator, and a solvent. However, most hard coating agents suffer from the disadvantages of poor flex resistance and impact resistance when it is intended to increase the crosslinking density of coatings for better hardness. The use of high molecular weight acrylic oligomers or flexible acrylate oligomers and monomers including ethylene oxide in their molecules contributes to improvements in the flex resistance and impact resistance of substrates but leads to low hardness.
Particularly, fluorine compounds and silicon compounds are mainly used to improve the scratch resistance and fouling resistance of films. Fluorine-containing polymers with low refractive index and fluorine-containing olefin copolymers exhibit extremely high water and oil repellency and expose —CF2— or —CF3 groups at the surface of films to ensure good antifouling properties, but are not readily formed into films by coating because they are hardly soluble in solvents. These polymers and copolymers are very expensive and undergo drastic changes in physical properties depending on molding conditions.
In addition to good heat resistance, polyimide resins have outstanding mechanical properties, including mechanical strength, wear resistance, dimensional stability, and chemical resistance, and excellent electrical properties, including insulation performance. Due to these advantages, polyimide resins are used in a wide range of industrial applications, including electrical and electronic applications.
Polyimides are widely used as highly heat resistant advanced materials, such as automotive materials and aeronautical materials, and in the field of electronic devices, such as interlayer insulating films for semiconductors, buffer coats, substrates for flexible printed circuit boards, liquid crystal alignment layers, and electrical insulating materials, because of their excellent mechanical properties, heat resistance, chemical resistance, and electrical insulation properties.
However, general polyimide resins have high transmittance in the visible region owing to their high density aromatic cyclic structure and are colored yellowish owing to their very low transmittance, particularly at wavelengths of around 400 nm, limiting their use in applications where transparency is required.
Colorless transparent polyimide resins can be used as raw materials for the production of highly heat resistant transparent coatings and films in the field of electronic devices, for example, materials for electrode insulating films and transparent protective films in the field of liquid crystal displays, hard coating films of touch panels, and hard coating films and transparent films in transparent flexible substrates.
Under these circumstances, continued efforts have been made to develop polyimides that can exhibit better chemical resistance and storage stability, sufficient mechanical properties, and excellent high temperature stability.